Along with the miniaturization of a semiconductor device, the trend is moving into shorter wavelength of the exposure light source and higher numerical aperture (high NA) of the projection lens. At present, an exposure machine with NA of 0.84 has been developed, where an ArF excimer laser having a wavelength of 193 nm is used as the light source. As commonly well known, these factors can be expressed by the following formulae:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are coefficients related to the process.
For more shortening the wavelength and thereby obtaining higher resolution, a so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been conventionally known as a technique for raising the resolution in an optical microscope.
As for the “effect of immersion”, assuming that NA0=sin θ, the above-described resolution and depth of focus in the immersion can be expressed by the following formulae:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 wherein λ0 is the wavelength of exposure light in air, n is the refractive index of the immersion liquid based on air, and θ is the convergence half-angle of beam.
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the depth of focus can be made n times larger by the immersion. This is effective for all pattern profiles and can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
An example of the apparatus where immersion exposure is applied to the transfer of a fine image pattern of a semiconductor device is described in JP-A-7-220990 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and the like.
The latest technical progress of immersion exposure is reported, for example, in Proc. SPIE, Vol. 4688, page 11 (2002), J. Vac. Sci. Technol., B, 17 (1999) and Proc. SPIE, Vol. 3999, page 2 (2000). In the case of using an ArF excimer laser as the light source, pure water (refractive index at 193 nm: 1.44) is considered to be most promising as the immersion liquid in view of safety in handling as well as the transmittance and refractive index at 193 nm.
With the advent of a resist for a KrF excimer laser (248 nm), an image forming method called chemical amplification is used as the image forming method for a resist so as to compensate for the reduction in sensitivity due to light absorption. To explain the image-forming method of positive chemical amplification by example, this is an image forming method of exposing a resist to decompose an acid generator in the exposed area to thereby generate an acid, utilizing the generated acid as the reactive catalyst to change an alkali-insoluble group to an alkali-soluble group by the bake after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
A resist for an ArF excimer laser (wavelength: 193 nm) using this chemical amplification mechanism is becoming predominant at present.
In the immersion exposure process, when exposure is performed using a scan-type immersion exposure machine, unless the immersion liquid moves following the movement of lens, the exposure speed decreases and this may affect the productivity. For this reason, it is preferred that the resist film has a high receding angle for the immersion liquid.
In JP-A-2006-048029, International Publication No. 07/116,664, JP-A-2008-65098, it is indicated that the resist film surface can be hydrophobed by adding a specific resin or the like.